With recent news of additional radioactive leaks from the Fukushima nuclear power plants, the radioactive effect on the ocean remains unclear. But a new study by U.S. and Japanese researchers analyzes the levels of radioactivity discharged in the first four months after the accident, drawing some basic conclusions about the history of contaminant releases to the ocean.

The study was conducted by Woods Hole Oceanographic Institution chemist Ken Buesseler and two colleagues based in Japan, Michio Aoyama of the Meteorological Research Institute and Masao Fukasawa of the Japan Agency for Marine-Earth Science and Technology.

They report that discharges from the Fukushima Dai-ichi nuclear power plants peaked a month after the March 11, 2011, earthquake and tsunami that precipitated the nuclear accident and continued through at least July.

Their study finds that the levels of radioactivity, while high, are not a direct threat but cautions that the effect of accumulated radionuclides in marine sediments is unknown.

The release of radioactivity from Fukushima—both as atmospheric fallout and direct discharges to the ocean—represents the largest accidental release of radiation to the ocean in history. Concentrations of radioactive isotopes 18 miles offshore were higher than those measured in the ocean after the Chernobyl accident 25 years ago. Buesseler said this is largely related to the fact that the Fukushima nuclear power plants are located along the coast, whereas Chernobyl was several hundred miles from the nearest salt water basins, the Baltic and Black Seas.

The study used data on the concentrations of radioactive isotopes as a basis to compare the levels of radionuclides released into the ocean with known levels in the sea surrounding Japan prior to the accident.

The resulting paper, “Impacts of the Fukushima Nuclear Power Plants on Marine Radioactivity,” was published in the journal Environmental Science & Technology.